In the case of a shaft which is driven by a pedal crank, the prior art discloses detecting the torque which is introduced into the shaft for the pedal crank. If the torque and the angular velocity of the shaft are known, the power which is introduced into the shaft can be simply determined as a product of the torque and the angular velocity. Torque detection apparatuses which allow the instantaneous torque to be detected are of particular interest. Arrangements of a bottom bracket with at least two pedal cranks, which are arranged on a common shaft in a rotationally fixed manner in each case, are also important, it being possible for the torque which is introduced by each of the two pedal cranks to be detected separately and for the torque which results in the shaft to be detected.
The prior art discloses, in particular, a bottom bracket with a torque sensor unit, with the bottom bracket comprising at least one pedal crank, a shaft which is connected to the at least one pedal crank in a rotationally fixed manner, and a torque detection apparatus for detecting a torque in the region of the shaft, with the apparatus comprising a first magnetization and a sensor which detects a change in the first magnetization on the basis of the torque introduced into the shaft. In this case, the shaft is in the form of a solid shaft, the change in the first magnetization is detected by means of a coil which is arranged next to the solid shaft. The torque is detected using the magnetostrictive effect according to which the occurrence of mechanical stresses, as occur when the torque is introduced into the shaft, can lead to a change in the magnetization, in particular if a magnetization is impressed on the material of the shaft. The detected change in the first magnetization can be used to draw conclusions about the internal voltages in the solid shaft, these in turn allowing a conclusion to be drawn about the torque which is introduced into the solid shaft. The prior art describes only the detection of the magnetic field which is introduced into the solid shaft and, in the case of a bottom bracket with two pedal cranks, does not distinguish between which torque the individual pedal cranks introduce into the solid shaft.
DE 103 01 610 A1 describes a first hollow body and a second hollow body which can be mounted on one another in a plurality of segments along a complementary configuration of the end faces. The two hollow bodies are separated by an air gap. A magnetic field passes through the two hollow bodies, this magnetic field changing as soon as the hollow bodies are rotated relative to one another. An arrangement of this type is hardly suitable for use as a shaft in a bottom bracket since the spatial resolution is limited by the number of segments and a change in the magnetic field can be influenced by a change in the distance between the two hollow bodies. It is also disadvantageous that the magnetic field is to be designed as powerful, and therefore the functioning of other, in particular electronic, components may be disturbed.
DE 196 09 981 A1 describes a bottom bracket for a bicycle, with a detection apparatus for, in particular, the torque. The detection apparatus comprises a torsionally rigid internal bearing shaft with in each case an interlocking and force-fitting receptacle for the two pedal cranks. The torsionally rigid internal bearing shaft is connected to a torsionally elastic shaft such that the torsion is transmitted to the torsionally elastic shaft in at least one direction of rotation. The rotation of the torsionally elastic shaft is detected by means of a Hall sensor. The detection apparatus is susceptible to faults, and furthermore the temperature and moisture dependency of the elastic properties of the torsionally elastic shaft has an unfavorable effect. A further disadvantage is that the operator has to expend considerable force in order to rotate the torsionally elastic shaft, and therefore only a reduced torque is available for driving the bicycle and the bicycle is sluggish overall.
EP 0 954 746 B1 describes an apparatus and a method for detecting a torque which is exerted on a bottom bracket shaft. To this end, two signal transmitters, which are axially spaced apart from one another and each transmit a first and a second square-wave signal, are arranged on the shaft. The distances between the flanks of associated square-wave signals can be used to draw conclusions about a torque if an average is taken over at least one full revolution of the shaft. One disadvantage is that only an averaged torque and not the instantaneous torque can be detected. In addition, it is difficult to detect very small torques since the manufacturing tolerances of the signal transmitters play a role.
U.S. Pat. No. 6,644,135 B1 describes a bottom bracket, in the region of which pressure sensors are arranged, the forces acting on the bottom bracket being determined from said pressure sensors. Torques can be detected at best indirectly.